This invention relates to electrodeposition and more particularly to the electrodeposition of refractory metal carbides.
For some time, man has recognized the advantages which greater hardness in metals offers. Harder metals, more resistant to wear, reduce the need for frequent, costly replacement of parts.
Abrasive wear afflicts all manner of machinery in which metal surfaces contact other surfaces. For example, erosive wear plagues metals exposed to high velocity gas streams carrying hard particles, as in coal gasification, or even the lower velocity, liquid-entrained coal particles in a slurry flowing through a pipeline. The wearing of metals is frequently aggravated by high temperatures which lead to simultaneous metal oxidation, particularly in the newer energy industries.
Several approaches to reducing wear have been taken. Chief among these has been the formulation of ever harder alloys, such as the newer ones based on cobalt. Another route has been to modify only surface properties, rather than the bulk of the metal. This has been done by covering the bulk metal with a coating of another alloy. Still another method has been to modify the surface layer of the metal either by diffusing other metals into the surface (metalliding), by ion implantation, or by laser melting.
It has long been recognized that refractory carbides possess precisely the desirable hardness missing from metals and are stable at fairly high temperatures. Nevertheless, such refractory carbides lack the desirable ductility of metals. Consequently there have been many attempts to combine the two in order to gain hardness combined with ductility. One well-known technique has been hard-facing, the incorporation of carbide particles into a bulk metal. Another has been to produce carbide coatings on metals. However, existing coating methods have not been entirely successful. Plasma spraying, which involves impinging the carbide powder on the surface to be coated, requires temperatures near 1500.degree. C., is line-of-sight, and tends to produce somewhat porous coatings. Chemical vapor deposition can be carried out by combining two reactive gases so that the carbide reaction product is produced as a coating. Much development work has been done on this process, but the coatings are usually quite thin. Further, neither plasma spraying nor chemical vapor deposition allows any control over the stoichiometry of the coating.
In previous studies of electrochemical reduction, carbides were deposited from melts containing alkali metal fluorides and B.sub.2 O.sub.3 ; the metal and carbon are introduced as the oxide and as carbonate, respectively. This method of electrolysis results in the formation of millimeter-size crystals on the walls of the graphite crucible which serves as the cathode. Analysis of these crystals shows that their composition varies with the metal oxide/carbonate ratio in the melt. However, no adherent coating of carbide is formed.
During the 1960's, Senderoff and Mellors showed that excellent coatings of the refractory metals could be electroplated from the ternary eutectic of (Li, Na, K)F by adding the metal as a complex fluoride, and plating between the appropriate metal anode and the cathode to be plated at 750.degree.-800.degree. C. "Coherent Coatings of Refractory Metals," Science, (1966) 153, 1475, is incorporated herein by reference. Dense, adherent, and ductile plates were obtained, and there seemed to be no upper limit to the plating thickness; in fact the substrate could be dissolved away to produce freestanding refactory metal objects. However, the inventors pointed out that not only halides other than fluoride, but also oxyanions, must be absent for the process to be satisfactory.
Since electrochemical reduction by previously developed methods did not prove to be an effective method of coating objects, and since Senderoff et al. did not discuss the electrodeposition of carbides, no basis existed for combining the teachings of these papers. Indeed, if one wished to adapt the Senderoff procedure to plating carbides, the prior art would be discouraging, since one would expect carbonates to interfere with metal deposition. In fact, initial attempts to introduce carbon by the anodic oxidation of graphite proved unsuccessful. When the melts were undried, gaseous fluoromethane was formed. In dried melts, carbon was inert. While these difficulties would not exist if carbon were introduced into the melt in the form of an alkali metal carbonate, it was believed, based on the work of Senderoff et al., that doing so would prevent successful plating by introducing oxyanions into the melt. Moreover, carbon is not the only possible product of carbonate reduction. For pure alkali metal carbonates, four reactions are possible: EQU CO.sub.3.sup.2- +4e=C+30.sup.2- ( 1) EQU CO.sub.3.sup.2- +2e=CO+20.sup.2- ( 2) EQU M.sup.+ +e=M.degree. (3) EQU 2CO.sub.3.sup.2- +10e=C.sub.2.sup.2- +60.sub.2.sup.2- ( 4)
For carbide formation to occur, reaction (1) is required. It was not known whether that reaction would be favored in a ternary fluoride melt.